He was destined to become King of Arnor after his father's death in T.A. 861, but there were constant quarrels between Eärendur's sons. A civil war erupted when Eärendur finally died. Amlaith, however, failed to overcome his two younger brothers, so the Kingdom of Arnor was split into three and Amlaith became the first King of Arthedain. (His brothers founded the kingdoms of Cardolan and Rhudaur).

His reign in Arthedain was largely a peaceful one, although border disputes with the realms of Cardolan and Rhudaur - especially over Amon Sûl and the Weather Hills - never fully ceased. Under his rule the capital was formally removed from Annúminas to Fornost.

Unlike his forefathers, he did not take a name in Quenya the "High Tongue", but instead took a name in Sindarin, as did all of his descendants.

Amlaith ruled for eighty-five years, and as succeeded at his death in 946 by his son, Beleg.